Kondo Effect at Molecule-Metal-Interfaces: Investigation by Angle-resolved Photoemission and Orbital Tomography

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In certain cases the occurrence of a Kondo effect is observed at interfaces between molecules and metallic surfaces. By the interaction of a localized state with the bath of conduction electrons this effect can increase the density of states at the Fermi level and thus crucially influence the transport and contact properties. Besides extensive STM and transport investigations on relevant model systems the fundamental relations, which lead to the occurrence of the molecular Kondo effect, in particular the nature of the involved single particle states and the local magnetic properties, are not completely clarified. In this respect angle-resolved photoelectron spectroscopy (ARPES) provides access to microscopic information, which is not or only very limited available by STM. Employing the angular information allows statements on the character and the symmetry of the involved localized states. In an ideal case molecular single particle orbitals can be reconstructed for organic adsorbate systems. Additional information is provided by investigating the dependence of the photoemission signal from the polarization direction and photon energy of the incoming light. Beyond this, the relevant model parameters can be derived from ARPES experiments, based on which a consistent picture of the microscopic relations in the Kondo model or Single Impurity Anderson Model (SIAM) can be obtained. Based on own previous work this project will systematically investigate the Kondo effect at molecule-metal interfaces by ARPES and in particular answer the following questions: (i) Which symmetry is associated with the many particle signals such as the Kondo resonance? What can we learn about the nature of these states from symmetries? The nature of the states involved in the occurrence of the Kondo effect is not yet clear and will be derived from the angular information in ARPES and from experiments with variable light polarization. (ii) Determination of the characteristic temperature- or energy scale from the temperature dependence of the Kondo resonance and core level spectra. The experimental determination of the temperature dependence of the photoemission intensities over a large temperature range provides access to the relevant characteristic energy scales. (iii) Can the relevant model parameters be systematically varied in practice? The description of molecule-substrate interface systems in the framework of the SIAM involves model parameters, which can be determined spectroscopically or by comparison to corresponding calculations. An external control of theses parameters allows manipulating the interface properties.